RE: [PATCH ppc64,aarch64,alpha 00/15] Improve backend constant generation

["Wilco Dijkstra" <wdijkstr at arm dot com>]

> Richard Henderson wrote:
> On 08/12/2015 08:59 AM, Wilco Dijkstra wrote:
> > I looked at the statistics of AArch64 immediate generation a while ago.
> > The interesting thing is ~95% of calls are queries, and the same query is on
> > average repeated 10 times in a row. So (a) it is not important to cache the
> > expansions, and (b) the high repetition rate means a single-entry cache
> > has a 90% hitrate. We already have a patch for this and could collect stats
> > comparing the approaches. If a single-entry cache can provide a similar
> > benefit as caching all immediates then my preference would be to keep things
> > simple and just cache the last query.
> 
> Interesting.  That's already more detailed investigation than I'd done.  I had
> no idea the queries were so clustered.  I assumed that the queries would be
> scattered across various passes, and so the various constants across the
> function would get checked in sequence.
> 
> I would be very interested in seeing those stats when you've done.

Caching improves average buildtime by 0.1-0.2% - your patch seems to be slightly
faster than caching just 1 query, so that suggests caching a few entries would be
beneficial.

However looking at the immediates that are generated by the loops, it's feasible
to avoid linear/quadratic search loops altogether. So I think a generic immediate 
caching scheme won't be useful for AArch64.

Wilco